In FA (Factory Automation), an industrial network system controls production facilities (such as motors, robots, and sensors). The industrial network system includes various slave devices each of which performs data collection and control of each production facility installed in a production line and a master device (PLC: Programmable Logic Controller) that centrally controls the slave devices. The industrial network system installed in a production line or a production site is connected to a host computer through an information-system network, and a system manager or an engineer can perform state monitoring or maintenance of each industrial network system with the computer.
Standards such as DeviceNet, CompoNet, and EtherCAT (registered trademark) are well known as a control-system network (field network) that connects the PLC and the slave devices. The case that instruments are directly connected to each other through an internal bus is also regarded as a network, and defined as Backplane Bus. On the other hand, standards such as EtherNet/IP are well known as the information-system network that connects the PLC and the host computer. Thus, a plurality of network or protocol standards are well known as the industrial network. Therefore, a plurality of kinds of networks having different protocols are mixed together in one network system in some cases. In such network systems, for the purpose of relay between different kinds of networks, it is necessary to arrange a data relay device (also referred to as a router, a communication coupler, or a relay) to perform a gateway (protocol conversion) or a routing.
A general gateway (protocol conversion) method in the industrial network system will be described. FIG. 1 illustrates a configuration example of the network system including the plurality of kinds of industrial networks. A topmost computer 10 and a PLC 20 are connected to each other through EtherNet/IP, and three slave devices 21, 22, and 23 are connected to the PLC 20 through Backplane Bus. The slave device 23 and a communication coupler 30 are connected to each other through EtherCAT (registered trademark), and three slave devices 31, 32, and 33 are connected to the communication coupler 30 through Backplane Bus. The slave device 33 is connected to a communication coupler 40 through DeviceNet, and three slave devices 41, 42, and 43 are connected to the communication coupler 40 through Backplane Bus.
In the configuration, in the case that the computer 10 transmits a message to the lowermost slave device 43, the message is routed through six kinds of networks in total as follows.
(1) computer 10→(EtherNet/IP)→PLC 20
(2) PLC 20→(Backplane Bus)→slave device 23
(3) slave device 23→(EtherCAT)→communication coupler 30
(4) communication coupler 30→(Backplane Bus)→slave device 33
(5) slave device 33→(DeviceNet)→communication coupler 40
(6) communication coupler 40→(Backplane Bus)→slave device 43
At this point, as illustrated in a part (1) of FIG. 9, the computer 10 produces and transmits a frame in which pieces of header information H1 to H6 of all network protocols of nodes from the computer 10 that is of a source node to the slave device 43 that is of a destination node are sequentially added to message information M. As illustrated in parts (2) to (6) of FIG. 9, the PLC 20, the slave device 23, the communication coupler 30, the slave device 33, and the communication coupler 40, which perform the relay in the network, sequentially decapsulate the received frame to be able to deal with the difference of the protocol.
Because the method can not only decapsulate the frame but also implement the gateway, advantageously a mechanism of a communication coupler that performs the relay in the network can be simplified. At the same time, the conventional method has the following problems.
First, it is necessary that the source node transmitting the message generates the pieces of header information on all the network protocols. Therefore, it is necessary to understand a high level of technical processing and format specifications of all the protocols in a frame generation function of the source node. In the case that the message transmitted from the source node receives a routing abnormality, it is necessary for a person in charge of maintenance to understand the format specifications of all the protocols in order to analyze where and which the abnormality is generated. Second, with increasing number of kinds of routed networks (encapsulated severalfold), the header information is enlarged to reduce a data area (a payload size in a packet) that can practically be used by a user. In an environment in which the networks are mixed together, a frame size is restricted according to the network having the smallest MTU (Maximum Transmission Unit). For example, for the frame size of about 500 bytes, when the six pieces of 30-byte header information need to be added as described above, about one third of the frame size, namely, 180 bytes are occupied by the header information, which leads to degradation of communication efficiency. Third, it is difficult to comply with a new protocol. Every time the new protocol emerges, it is necessary to modify the frame generation function of the destination node, and to add the function of generating the header information on the new protocol and the function of the encapsulating the header information on the new protocol. Therefore, development man-hour increases to influence a wide range (a range of the instrument or program to be modified).
For example, Patent Document 1 to Patent Document 3 disclose the gateway or routing function in a general IP network. However, the above problems cannot be solved even if the technologies disclosed in Patent Document 1 to Patent Document 3 are diverted to the industrial network.